This invention is concerned with processes and apparatus for the extraction of at least one valuable metal from materials such as composite mixtures of metallic compounds which may contain relatively small amounts of such valuable metal.
In particular the invention is concerned with techniques and apparatus suitable for extraction of at least one valuable metal such as one or more of: gold, platinum, silver, cobalt, nickel, molybdenum, and manganese from a starting material which, by way of example, could be freshly mined mixed ore containing both oxidic and refractory (sulphide) ores. The starting material could be subjected to valuable metal extraction in the form of an aqueous slurry containing such material. If required, the starting material e.g. mixed ore can be crushed and/or ground to appropriate particle size before treatment. The starting material could, during the course of extraction, become mixed into an aqueous slurry if it is desired to commence an extraction process upon solid particulate material.
However the present invention is not limited to the use of such dry or wet mixed ores as the starting material. It will become apparent that the present invention can be used to treat other material which comprises metallic oxide and/or metallic sulphide in addition to the said valuable metal whether the valuable metal is present in the form of metal or metallic compounds.
In the extraction of valuable metals, such as listed above the metals may be contained in matter such as tailings, slimes, calcined ores, and other discarded materials which have already been processed in some way, but which it is now economically viable to extract.
It has been commonplace for freshly mined ores, and material from mine dumps which are to be further treated, to be transported for comparatively large distances, such as several miles, to a processing plant. The ore or dump material is usually transported as a slurry through suitable pipelines. When the slurry reaches the processing plant it is subjected to valuable metal-leaching and subsequent extraction. The overall cost of pipelines, leaching and extraction apparatus is high, especially as the leaching process usually required elevated temperature and pressure.
Although the possibility of carrying out reactions such as leaching within the interior of a pipeline as a reactor vessel has been previously proposed, practical pipeline reactors for carrying out such pressure reactions have not been generally available until pipeline reactor vessels termed xe2x80x98Hydrocoilsxe2x80x99 were made available. Such tubular pressure reactors are commercially available from the company, HMC Technology Limited.
For a description and drawing of such a tubular pressure reactor, the reader is referred to GB-A-2 158 734, the contents of which are incorporated herein, by reference. This prior reference also discloses by way of background a technique for extracting gold and other valuable metal from refractory ores by use of a leaching agent based on acid as an oxidising agent and sodium chloride as a source of halide ions. For further background information on extraction processes, the reader is also referred to prior specification EP-A-0 124 213, also incorporated herein by reference, wherein use of such a leaching agent is described in more detail.
The present invention represents a significant technical advance upon the procedures referred to in these prior patent specifications, in that it seeks to provide an economic and yet convenient method of carrying out a sequential two-stage or combined single stage leaching process for the extraction of gold or other valuable metal from starting material e.g. mixed oxidic and refractory ore which hitherto has not been amenable to cyanide extraction.
In addition embodiments of the present invention can be operated so as to overcome a substantial technical prejudice in the art, against valuable metal extraction using cyanide in the presence of acid.
Firstly gold cyanide complex loading on carbon column retaining means is severely hindered in the presence of base metals, which preferentially compete with the gold complex for available carbon loading sites. The oxidic copper content in prior ore material treatments, for example, has been so high that the amount of carbon required in terms of retention columns rendered such a process uneconomic. However, we have demonstrated that base metals if present in solution will not load to any significant extent on such carbon retention means at the values of pH we prefer in our present extraction processes i.e. less than 4.0, preferably less than 3.0 but more preferably less than 2.0 pH.
Secondly there are not unreasonable safety/toxicity fears surrounding acidic conditions for cyanide treatments, with potential formation of poisonous hydrogen cyanide gas. Despite these technical prejudices our extraction conditions include the presence of acid, preferably nitric with sulphuric acid (a known catalyst) and an aqueous source of halide e.g. a saline solution, thus providing favourable conditions for conversion of hydrocyanic gas (if such should form or tend to form) to the soluble cyanide ion in solution. This is further aided by the process being preferably operated under pressure e.g. as a result of pump pressure and/or gas generation.
In a preferred embodiment of the process, in the acid treatment stage 1 the nitric acid leach converts sulphides to sulphates but also produces sulphuric acid (which is why the pH can stay low even after oxide(s) of nitrogen NOx""s are flashed off)xe2x80x94hence stage 1 acid treatment and stage 2 cyanide treatment interlink so well. It may be necessary in stage 2 cyanide treatment to adjust the pH with slightly more sulphuric acid or alternatively making the reaction medium more alkalinexe2x80x94but stage 1 is in such an embodiment already producing some sulphuric acid in solution, which is desirable.
Such processes according to the invention can achieve and maintain a beneficial stability, with the minimum production of hydrogen cyanide gas.
According to this invention we provide a process for extracting at least one valuable metal from starting material comprising a plurality of metallic compounds including:
(i) said at least one valuable metal and/or compound thereof,
(ii) oxide of at least one other metal, and optionally
(iii) sulphide of the same or different other metal,
which process includes treating the starting material with liquid. leaching agent containing
(iv) acid and a source of halide ions, and with
(v) the same or different liquid leaching agent containing a source of cyanide ions,
so as to solubilise said valuable metal(s)
whereby liquor obtained from the treatment with leaching agent which contains said solubilised valuable metal(s), is caused to contact valuable metal retaining means wherein said valuable metal(s) is(are) retained.
Subsequently it is preferred to release the retained valuable metal(s) from said retaining means. This can be achieved by techniques which are well known in the art.
In a second aspect the invention also provides apparatus for carrying into effect the above process. Such apparatus is illustrated and described subsequently hereinafter, with reference to embodiments of the present invention.